This invention refers to a flexible, intermediate, bulk container made of coated or uncoated woven fabric or plastics film having antistatic properties. The container is equipped with elements which are enabled for corona discharge of static electricity accumulating in said container.
Flexible, Intermediate, Bulk Containers are specified in European Standard EN 1898, which was approved by CEN on 15 Jun. 2000 and which is incorporated herein by reference. In this European Standard, it is mentioned that such FIBCs may be subject of special electrostatic conductivity treatment, however, there is no further statement about the generation of electrostatic charges and advantageous designs which reduce the risks resulting from such generated electrostatic charges.
A flexible, intermediate, bulk container (FIBC) is also disclosed in U.S. Pat. No. 5,071,699. Segregation processes of moving product particles as well as segregation created between the product particles and the FIBC during filling and emptying of the FIBC create localized pockets of built-up static electricity in the FIBC. Incendiary discharges from the charged FIBC can be dangerous when combustible dust arises inside the FIBC and/or in a hazardous area with explosive dust-air mixtures or explosive gas/vapour/mist-air mixtures, and can be quite uncomfortable to workers handling such containers. To avoid these disadvantages, it is suggested that the woven fabric of the FIBC should contain a plurality of interwoven quasi conductive filament fibers. The purpose of the quasi conductive fibers is to more evenly distribute the electrostatic charges which may build up on the surfaces and to effect corona discharges at the ends of the filament fibers. Preferably, the fibers are interwoven at regular intervals so that they are evenly spaced apart across the surface of the fabric. Such FIBCs need not be grounded during filling and emptying operations. As static charges are generated, the electrons can bleed into the atmosphere. Dangerous electrostatic charges are reduced but not eliminated.
A similar approach is disclosed in U.S. Pat. No. 5,458,419. An FIBC is equipped with a grid of interconnected conductive filaments and can be grounded via a conductive grounding tab and/or the conductive lifting loops. The FIBC must be grounded during filling and emptying to discharge and to eliminate the dangerous electrostatic charges. However, grounding is additional work which should be avoided and, if the grounding is improperly done, risks from static electricity still prevail.
The bleeding effect of electrons into the atmosphere is known to experts as corona discharge. Several types of discharge are distinguished in electrostatics on a purely phenomenological basis, that depends on the conductivity and geometric arrangement of the charged objects. This distinction is of great significance for industrial practice in as much as each type of gas discharge exhibits a different incendivity towards flammable atmospheres. Generally, four types of discharge exist:    spark discharge,    brush discharge,    propagating brush discharge, and    corona discharge.
The corona discharge can be understood as a special case of a brush discharge. If the radius of curvature of the grounded electrode which is introduced in a powerful electric field is very small, for example less than 1 mm, the field is disturbed only in the immediate vicinity of the pointed electrode. This gives rise to a very weak gas discharge restricted to the immediate vicinity of the point, which, in contrast to a brush discharge, is not triggered abruptly and does not lead to visible discharge channels. Depending on the quantity and replenishment rate of the charge carriers that generate the field, a corona discharge shows a more or less constant discharge over a lengthy period of time and hence must be regarded as a continuous gas discharge.
The quasi conductive fibers interwoven with the fabric of the FIBC disclosed in the a.m. prior art collect the locally accumulated electrostatic charges. The electrostatic charge now contained in the quasi conductive fibers is transmitted to their discharge points which are at the ends of the fibers. At these ends the corona discharge mainly occurs. The disadvantage of the containers known from prior art is the relatively long time period which is required to achieve a neutralized charge status at the ends of the quasi conductive fibers. For some applications it takes too long before a high electrostatic charge is eliminated by corona discharge at the ends of the quasi conductive fibers interwoven with the fabric.